Since the advent of emissions standards for internal combustion engines for cars, trucks, and other vehicles, emissions of hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) have markedly declined. This decline has been brought about through the use of a variety of techniques including electronic fuel injection, electronic engine control, and the use of a variety of catalytic converters to oxidize HC and CO, and to reduce NOx to nitrogen. However, increasingly more stringent emissions standards require even less pollutants in the gas emissions over extended periods of engine operation.
With these tighter emissions requirements, it is essential to form a catalyst capable of reaching its minimum operating temperature almost immediately upon starting the engine. Close-coupled catalytic converters, which are located in the engine compartment, i.e., beneath the hood and adjacent to the exhaust manifold, are of prime investigative interest as the principal function of a close coupled catalyst is to reduce hydrocarbon emissions during the cold start phase, which is defined as the period immediately after staring the engine from ambient conditions, and which last for about 2 minutes. The cold start period depends on the ambient temperature, the type of engine, the engine control system, and engine operation.
An inherent problem typically associated with close coupled catalytic converters is their tendency to quickly corrode, a result of their exposure to high temperatures, e.g., over about 1,000xc2x0 C., to which the converters are exposed. Close coupled catalytic converters typically contain a catalytic material deposited on a support. The catalytic material, such as, for example, palladium, is preferably in the form of an oxide rather than the pure metal, as oxides have greater catalytic activity.
However, at temperatures of about 800xc2x0 C., temperatures that are easily reached in close-coupled catalytic converters, the palladium oxide, for example, decomposes to palladium, which is catalytically less active than are the oxide forms. Consequently, HC, CO, and hydrogen (H2) decomposition greatly decreases when the palladium oxide is decomposed to palladium metal.
Current problems also exist with regard to the slight attraction that typical supports, such as lanthanum oxide and aluminum oxide, exhibit towards palladium and rhodium metal and oxide forms. Due to the slight attraction, the catalytic material is capable of migrating across the support, and then forming dispersed agglomerated particles, thereby reducing the surface area of the palladium and rhodium. This migration is even more drastic at temperatures in excess of about 1,000xc2x0 C. As catalytic activity is dependent on the exposure of a large surface area of the catalytic material, the formation of these dispersed agglomerated particles decreases the exposed surface area by about 10 times or greater, and, hence, decreases catalytic activity. In addition, at temperatures exceeding about 1,000xc2x0 C., typical supports, such as lanthanum oxide, aluminum oxide, and the like, have only a slight attraction for palladium oxide and rhodium oxide, and even less attraction for palladium metal and rhodium metal. Therefore, there exists a need to increase the efficiency of catalytic converters by preserving the life-span of those catalytic materials necessary for the reduction of pollutants in gas emissions.
Disclosed herein is a gas treatment device comprising a catalyst composition disposed on a substrate to form an SMSI-coated substrate, wherein the catalyst composition includes an SMSI material and a support; a shell disposed around the SMSI-coated substrate; and a retention material disposed between and in physical communication with the substrate and the shell; wherein the gas treatment device can withstand temperatures up to about 1,150xc2x0 C.
Further disclosed herein is a method for forming a close-coupled catalytic converter comprising forming a slurry comprising an SMSI material and a support; applying the slurry to a substrate to form a coating; calcining the coating to about 1,000xc2x0 C. in a water-containing atmosphere to form an SMSI-coated substrate; disposing the SMSI-coated substrate into a housing; disposing a retention material concentrically between and in physical communication with the housing and the SMSI-coated substrate to form a unit; and positioning the unit into the close-coupled position of a stoichiometric gasoline engine.
The above described and other features are exemplified by the detailed description.